8 seconds or less
Often local wind sea: waves are closer together, less organized, and more affected by nearby wind.
SwellOracle Academy
What is swell period?
Period is the time between one wave crest and the next. It helps reveal how organized a swell is and how it may behave as it reaches the coast.
What the number measures
Period is measured in seconds. If successive crests pass a buoy 14 seconds apart, that swell has a 14-second period. A buoy may report a dominant period for the strongest energy band and an average period across the sea state.
Always read period together with swell height and direction. Period alone cannot tell you the size or quality of the surf at a particular beach.
A practical reading guide
9–12 seconds
A medium period. It may be improving windswell, a regional swell, or a mixture of wave systems.
13–16 seconds
Usually a more organized groundswell with wider spacing and more noticeable sets.
17 seconds or more
Long-period energy from a distant source. It can arrive in powerful, widely spaced sets.
Same height, different ocean
Two swells can have the same offshore height but produce very different surf. Longer-period waves are spaced farther apart and begin interacting with the seabed in deeper water, so reefs, points, and bays can focus or filter them differently.
3 ft @ 8s
Shorter spacing and usually less organized. The sea may feel busy, with more frequent but weaker waves.
3 ft @ 14s
More organized sets and greater potential to wrap into exposed points and reefs.
3 ft @ 17s
Long gaps can hide the power between sets. Expect stronger surges and larger variation from lull to set.
Longer does not automatically mean better
A long period is not a quality score. A beach still needs the right swell direction, enough height, suitable wind, tide, and local bathymetry. Some spots become excellent with long-period swell; others close out or stay sheltered.
The period ranges above are useful clues, not universal rules. Local knowledge and live observations should always complete the forecast.
How to use period in SwellOracle
First pair the reported period with its height and direction. Then compare nearby buoy observations with the model point for your coast. A rising period can be an early sign that a new swell is arriving, even before the largest sets show at the beach.
Finally, check the timestamp: an old buoy reading should not outweigh a recent observation or what you can see at the coast.
Practical takeaway
Period is context, not a verdict: combine seconds with height, direction, wind, tide, and the way your spot receives swell.